Botched ritual

Soul magic is not something you can play around with. It's not like charms that you can cancel with finite, or like curses that you can counter with appropriate counter-curse. It always involve ritual, obviously. Rituals are not something you can joke around either.

Lily Potter knew that.

Unfortunately, since Harry was born, she had often than not nightmares. At first she thought that stress is finally getting to her. That the risk of her baby life is getting to her and it's normal to have nightmares with such predicament. Then nightmares started to be remote persistent. Until it boiled down to the same dream: Voldemort found them, she run upstairs, few words "step aside" and green light of killing curse.

It was driving her crazy, she was driving James and boys crazy. They started to avoid her. So she decided that if this dream is a glimpse of the future, she can work with it.

Hence a ritual.

She hide ritual circle under baby crib.

She added necessary runes on her body and on Harry head. Of course to avoid James questions - she covered it with glamour.

The goal was to exchange her soul for enemy soul. Basically kill who ever killed her.

Simple like that.

And if her dreams are true glimpses of the future then fuck you Voldemort from mudblood witch.

—-

On Halloween night when Voldemort get vanquished soul magic based ritual got botched. Lily Potter did not know that Voldemort was doing his own soul magic ritual of creating horcrux.

As a result piece of Voldemort soul got ruined and replaced with Lily soul and all that got stuck in rune scar on a little Harry head.

—-

17 years later, when not so little Harry followed Dumbledore plan to get killed by Voldemort, botched ritual finally got completed.

Soul magic is not something you can play around with.